The following publications are representative of the most relevant prior art known to the Applicants at the time of filing of the application.
______________________________________ UNITED STATES PATENTS ______________________________________ 2,194,472 March 26, 1940 G. H. Jackson 2,216,728 October 8, 1940 R. C. Benner et al 3,183,071 May 11, 1965 C. V. Rue et al 4,364,746 December 21, 1982 D. Bitzer et al ______________________________________
Of the many metals and alloys of commercial importance, titanium metal and its alloys have been the most troublesome to process at various stages of fabrication into a finished product. One of the most serious problem areas is the grinding of titanium and its alloys. Titanium is an extremely difficult material to machine and grind. Titanium is highly susceptible to oxidation i.e. it's very reactive with oxygen, especially at elevated temperatures such as those created during grinding of the metal. The oxidation reaction is highly exothermic thereby generating a substantial amount of heat which is additive with the normal heat of grinding experienced grinding any metal. To compound the problem titanium has a relatively low thermal conductivity as compared with the ferrous metals which results in a greater concentration of heat at the grinding surface.
This high concentration of heat at the grinding surface, i.e. the interface between the grinding wheel and the metal being ground, raises havoc with the grinding wheel or other abrasive tool being used. If the abrasive product's bond is an organic polymer as would be found most likely in a snagging wheel or a coated abrasive product, this extreme thermal condition will cause premature thermal decomposition of the organic polymer bond, resulting in costly high abrasive product usage. In addition, the high temperature and highly chemically reactive environment accelerates wear of the abrasive being used, particularly if the abrasive is an oxide based abrasive such as alumina or cofused alumina-zirconia. Besides these thermally caused problems, snagging of titanium involves relatively high grinding forces. The mechanism of shear of titanium results in a higher force and a thinner chip per unit depth of cut as compared to ferrous metals; this naturally results in higher sliding speed and higher unit forces on the abrasive grains. Lower grinding speeds required by the thermal conditions inherent in the grinding of titanium create higher shear stresses during grinding.
To the present time, the grinding of titanium still remains an undesirably expensive step in the fabricating of titanium parts. The most effective abrasive grain for the conditioning of titanium is a cofused alumina-zirconia abrasive containing about 25% by weight of zirconia; its reasonable success is due to its superior toughness as compared to all other abrasives. However, because it is an oxide, the alumina-zirconia abrasive is very susceptible to reaction with the titanium at the heat of grinding. Conventional crystalline silicon carbide is also effective in grinding titanium as long as the grinding operation does not require high grinding pressure or forces; in this case the silicon carbide single crystal abrasive grains are too weak to perform efficiently. The silicon carbide does have the advantage of being resistant to dissolution in the hot titanium because it is not an oxide.
While the foregoing is the current status of the abrasives used to grind titanium, there are other types of abrasives known in the industry but not known to have been used successfully in grinding titanium.
U.S. Pat. No. 2,194,472 discloses an abrasive wherein each abrasive grain is not a single crystal or particle but rather is an agglomeration or aggregate of a number of relatively small abrasive particles bonded together. Specifically, as it relates to the present invention, the reference teaches bonding 50 and/or 280 grit silicon carbide abrasive with a clay by mixing the two materials, firing at 1250.degree. C. to vitrify the clay, cooling the mass and finally breaking up the mass and screening the particles to the desired size. The aggregate particles are then used to make a coated abrasive product. There is no discussion with respect to how the coated abrasive product is to be used, except by implication in that the article is an abrasive articles.
Prebonded abrasive aggregates are taught by U.S. Pat. No. 2,216,728 wherein the aggregates are made up of a plurality of smaller grains of diamond or boron carbide held in the aggregate by a bond which may be a metal, clay, glass or an organic polymer. The method of formation of the aggregates will vary slightly depending on the nature of the bonding medium employed. If metal is the bond then the metal powder and fine abrasive particles, e.g. diamond, are mixed together and hot pressed at a temperature of from 700.degree. to 1500.degree. depending on the metal used. If the abrasive is diamond the hot pressing is done in an non-oxidizing atmosphere. Ceramic bonded aggregates are made by mixing about 5% clay with 95% fine abrasive grain with the usual liquid to give the mixture the needed consistency. The mix is then fired at for example 1250.degree. to vitrify the clay bond. After cooling, the bond mass is broken up into the desired particle size; these particles are then bonded into a grinding wheel with additional bond usually of a different type than the bond used to form the aggregates.
A heavy duty snapping abrasive is disclosed in U.S. Pat. No. 3,183,071 which is made up of bonded particles of very fine crystalline alumina having a particle size of less than 5 microns. Abrasive pellets of various cross sections are formed by extruding mixtures of fine alumina particles and a bond, cutting the extrudate at the desired size, and firing the green pellets. The bond is a silicate glass which has a final fired weight composition of 10-25% alumina, 50-70% silica, 5-15% calcia, 10-20% magnesia, and up to about 3% impurities. The fired pellets are bonded into a grinding wheel and use to snag grind stainless steel.
U.S. Pat. No. 4,364,746 adds a further variant to the technology of agglomerated abrasive grain. Prebonded abrasive aggregates in this case, are made up of fine particles of an abrasive material such as alumina or silicon carbide bonded into the larger abrasive particles by a resin or polymer. Aggregate particles of different strengths are made by incorporating various types and amounts of filler materials in the resin or polymer binder used to hold the fine abrasive particles together to form the larger abrasive agglomerates. In addition to incorporating fillers in the fine grain binder, the properties of the final aggregate are also varied by using different amounts and different types of resin as the binder. These aggregates of various strengths are blended in a given coated abrasive product or grinding wheel to give a particular desired end result.
The disclosed abrasive aggregates are also utilized with the more conventional type abrasive grains such as fused crushed alumina, alumina-zirconia and the like, including silicon carbide, boron carbide and the diamond.